How to Say Worth It / Worthed Correctly?

Which one is the correct word? And how to use it in a proper way?
a. Worth it
b. or, Worthed

There is no such word as : “worthed”. We can’t say “worthed” as past participle of “worth” since “worth” itself is not a verb.

From dictionary.reference.com/browse/worth, some formal examples of how to use “worth” in sentence:

As preposition
– having a value of, or equal in value to, as in money : This laptop is worth 5 million rupee
– having property to the value or amount of : They are worth of millions

As noun
– usefullness or importance : Your worth to the world is inestimable

So, which one is better to be said:

It is not worth to buy

It is not worthed to buy

It is not worth it to buy

The correct answer is sentence number 3.
Notice that “it” (in “worth it”) has a function as noun replacement:
– This laptop is worth 5 million rupee
– This laptop is worth (it) –> however, the speaker has to warrant the context of “it” here.